The quality of microfilms varies greatly from low to high depending on the production conditions, the kind of film, etc. It is difficult for users in general to judge the quality of a particular microfilm and determine the proper amount of exposure by observing the reduced image on the film. In the case of the reader mode wherein the microfilm image is projected on a screen on enlargement, the user can adjust the film image to an easily viewable brightness by controlling the quantity of light of the illuminating lamp, but when in the printer mode, it is extremely difficult to determine the proper amount of exposure for the photoconductive member.
Accordingly, reader-printers have been proposed which have a photodetector disposed in a light path for detecting the microfilm image to determine the proper amount of exposure as disclosed, for example, in U.S. Pat. No. 4,433,906. With the disclosed technique, the single photodetector is provided at an intermediate portion of the light path to receive unfocused image-forming rays incident thereon. However, it is likely that the arrangement fails to determine the amount of exposure with high precision, for example, when the film has an uneven distribution of densities.
The present applicant has therefore proposed in Japanese Laid-Open Patent Application No. SHO 58-187922 a system wherein a plurality of photodetectors are arranged in an optical path, and an optimum amount of exposure is determined from a maximum and a minimum of quantity of light received by these photodectors. With this system, each photodetector is connected to a comparator, and the comparison voltage to be applied to the comparator is varied stepwise. The photodetector has its state changed when the varying comparison voltage reaches a level corresponding to the quantity of light received. Accordingly, the comparison voltage which causes one of the photodetectors to change its state first and the comparison voltage which finally causes the change of state in the last one can be detected by checking the photodetectors for the change of state, and the proper amount of exposure can be calculated from appropriate equations with use of the voltage values. The calculation equations are prepared specifically for negative films, as well as for positive films. The system provides optimum amounts of exposure for films of very poor to good qualities.
With the above system, identical stepwise variations of comparison voltage are used for preparing positive copies from negative films (hereinafter referred to as "N.fwdarw.P") and for producing positive copies from positive films (hereinafter referred to as "P.fwdarw.P"). Amounts of exposure can be determined even when the variations in the comparison voltage level are common for the two cases. However, the precision of measuring the amount of light is lower through positive films than through negative films, because the positive film has a larger blank area (transparent area) than the negative film, permitting all the comparators to give an output at a lower comparison voltage than is the case with the negative film, and further because the photodetector has a relatively larger area than characters and is therefore greatly influenced by blank portions even when receiving the light through image areas. For example, when a positive film and a negative film which are usual and approximately the same in the ratio of characters are compared, the comparison voltage at which one of the comparators gives an output first is much lower for the positive film than for the negative film, and even the comparison voltage causing all the comparators to produce an output for the positive film is lower than the comparison voltage for causing one of the comparators to produce an output first for the negative film. Experiments conducted on a large number of films of different qualities have shown that on the average the comparison voltage for causing all the comparators to give an output for positive films is about 1/3 of the corresponding voltage for negative films.
Thus, although the range of variations of the comparison voltage is different for different types of films, the comparison voltage level is varied stepwise by the same amount at each step for the different types. This means that the number of steps in the variation range for the positive film is exceedingly smaller than the number of steps in the variation range for the negative film. In other words, the light through the positive film is measured with lower precision relative to the variation range than the light through the negative film.
Images are recorded on microfilms in different orientations with respect to the length or width of the film, depending on the contents of the record. Accordingly, reader-printers are generally so adapted that the size (as well as the orientation) of paper is selectable. With some reader-printers, the magnification is also selectable. When the above auto exposure control system is used for such a reader-printer which is designed for the selection of paper size and the magnification, it is likely that some of the photodetectors arranged in the optical path will not receive the light through the image on the film but will be exposed to the light through a portion other than the image area, depending on the image size of the film. When auto exposure control is effected in such a state, the output of the photodetectors corresponding to the above-mentioned portion other than the image area will be processed also as information as to the film image, with the resulting likelihood that a satisfactory copy will not be obtained owing to a noise contained in the amount of exposure calculated.
The problem may be overcome by arranging photodetectors only for the overlapping portions of all the image sizes selectable, but this still fails to provide a basic solution because satisfactory copies are not available from images of large size which has an uneven distribution of densities.
Thus, the foregoing system has the drawback of being not always capable of determining amounts of exposure fully accurately because of the difference between positive and negative, the difference in size or like difference.